The invention relates to electrolyzers for the production of magnesium and more specifically to a new cathode unit suited for the building up of stable cathode constructions with a number of different configurations for use in such electrolyzers.
The well-known I.G. electrolyzers for the production of magnesium consist of a number of cells formed by one anode with vertical surfaces and two cathodes, each with an active electrode surface facing the anode. The chlorine gas and metal produced are maintained separate by means of special curtains, so-called diaphragms, which divide each cell into zones for collecting chlorine and magnesium, respectively. Therefore an electrolyzer having a plurality of cells has a number of gas- and metal-collecting zones.
To increase the effective or working surfaces of the electrode elements and also to increase the output of metal from each electrolyzer unit, it is known to arrange transverse cathode components running to the anode zone. Such transverse cathode elements are preferably arranged in parallel in relation to each other, at the same distance from each other and extending at right angles in relation to the main cathodes in such a way that that there will be formed closed cathode loops or frames enclosing the anodes. A closed cathode frame of such configuration is known from and described for the first time in U.S. Pat. No. 3,496,089.
However, the development has lately taken a direction towards larger and more efficient types of electrolyzers having double acting cathodes, wherein the gas from the cells is collected in a common zone and wherein the natural flow is utilized also to transport metal and electrolyte from each cell to a common separation and collection zone for metal. By the term "double acting cathodes" is thus meant cathodes where both side surfaces are electrolytically active and where each active surface faces a complementary anode surface. This means that anodes and cathodes are arranged alternatively one after the other in a row. Electrolyzers which work according to this principle comprise two main zones or areas, one electrolysis zone and one metal separation zone, which are separated by means of a partition wall. The partition wall has openings at the lower end for flow of electrolyte to the electrolysis zone and openings at the upper end, near the electrolyte surface, through which openings electrolyte and the magnesium metal formed will flow into the metal zone.
The known cathode frame principle with longitudinal and transverse cathode elements is also utilized in this more modern type of electrolyzer cell, such as, for example, described in U.S. Pat. No. 3,907,651, where the longitudinal hollow cathodes are connected by means of transverse steel plates welded to the cathodes, in such a way that there is formed a closed loop or frame enclosing each anode.
A drawback associated with the use of such closed cathode frames where two or more parts of the frame are in close contact with the electrolyzer wall, is that due to the high working temperatures and the limited possibilities for heat expansion there will be generated high strains in the material and deformation of the cathodes. Attempts have been made to solve this problem by reinforcing the cathode frames, but this has not, however, been a satisfactory solution.
Furthermore the frames are heavy and troublesome to handle in connection with the mounting or assembling of the electrolyzers and also in connection with the dissassembling of the same. Generally speaking, an extensive prefabrication of the components used for the assembly of electrolyzers is highly desirable. Concerning the use of cathode frames, such prefabrication has so far not been realized, as there have been no means of securing accurate mounting and the correct anode/cathode spacing. During the manual assembly and bricklaying which today is performed, it will therefore be necessary to weld the cathode frames during the assembly of the electrolyzer cells, which additionally will complicate the utilization of cathode frames in magnesium electrolyzers.